On 2021-10-12, Jakub Jelinek wrote:
On Tue, Oct 12, 2021 at 09:21:21AM -0700, Fāng-ruì Sòng wrote:
> > An output constraint takes a lvalue. While GCC happily strips the
> > incorrect lvalue to rvalue conversion, Clang rejects the code by default:
> >
> > error: invalid use of a cast in a inline asm context requiring an
lvalue: remove the cast or build with -fheinous-gnu-extensions
> >
> > The file appears to share the same origin with gmplib longlong.h but
> > they differ much now (gmplib version is much longer).
> >
> > I don't have write access to the git repo.
> > ---
> > include/longlong.h | 186 ++++++++++++++++++++++-----------------------
> > 1 file changed, 93 insertions(+), 93 deletions(-)
> >
> > diff --git a/include/longlong.h b/include/longlong.h
> > index c3e92e54ecc..0a21a441d2d 100644
> > --- a/include/longlong.h
> > +++ b/include/longlong.h
> > @@ -194,8 +194,8 @@ extern UDItype __udiv_qrnnd (UDItype *, UDItype,
UDItype, UDItype);
> > #if defined (__arc__) && W_TYPE_SIZE == 32
> > #define add_ssaaaa(sh, sl, ah, al, bh, bl) \
> > __asm__ ("add.f %1, %4, %5\n\tadc %0, %2, %3" \
> > - : "=r" ((USItype) (sh)), \
> > - "=&r" ((USItype) (sl)) \
> > + : "=r" (sh), \
> > + "=&r" (sl) \
> > : "%r" ((USItype) (ah)), \
> > "rICal" ((USItype) (bh)), \
> > "%r" ((USItype) (al)), \
>
> This seems to alter the meanining of existing programs if sh and sl do
> not have the expected type.
>
> I think you need to add a compound expression and temporaries of type
> USItype if you want to avoid the cast.
Add folks who may comment on the output constraint behavior when a
lvalue to rvalue conversion like (`(USItype)`) is added.
Allowing the casts in there is intentional, the comment about this
e.g. in GCC's C FE says:
Really, this should not be here. Users should be using a
proper lvalue, dammit. But there's a long history of using casts
in the output operands. In cases like longlong.h, this becomes a
primitive form of typechecking -- if the cast can be removed, then
the output operand had a type of the proper width; otherwise we'll
get an error.
If you try e.g.:
void
foo (void)
{
int i;
long l;
__asm ("" : "=r" ((unsigned) i));
__asm ("" : "=r" ((long) l));
__asm ("" : "=r" ((long long) l));
__asm ("" : "=r" ((int) l));
__asm ("" : "=r" ((long) i));
}
then on e.g. x86-64 the first 3 asms are accepted by GCC, the last two
rejected, because the modes are different there.
So the above change throws away important typechecking. As it is
used in a macro, something different should verify that if the casts are
removed.
Jakub
Thanks for the detailed explanation. I see the type checking effect now.
In many cases the assembler can provide some checks, e.g.
long foo(long a) {
long r;
__asm__ ("movq %1, %0" : "=r"(r) : "r"(a));
return r;
}
I came here from trying to build glibc with Clang where I came across errors
like
In file included from strtof_l.c:44:
./strtod_l.c:1500:26: error: invalid use of a cast in a inline asm context
requiring an lvalue: remove the cast or build with -fheinous-gnu-extensions
udiv_qrnnd (quot, n, n, 0, d);
~~~~~~~~~~~~~~~~~~^~~~~~~~~~~
./longlong.h:518:24: note: expanded from macro 'udiv_qrnnd'
"=d" ((UDItype) (r)) \
~~~~~~~~~~~^~
In file included from strtof_l.c:44:
./strtod_l.c:1606:21: error: invalid use of a cast in a inline asm context
requiring an lvalue: remove the cast or build with -fheinous-gnu-extensions
add_ssaaaa (n1, n0, r - d0, 0, 0, d0);
~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
(Seems that building GCC with Clang doesn't run into the issues?)
If the desire to keep the checking is still strong, perhaps I will just stick
with using the -f option for glibc.
